The overall objective is to test the hypothesis that initial pathogen reproduction at the outbreak focus is the primary determinant of subsequent epidemic spread for pathogens with "fat-tailed" dispersal kernels, which have potential for significant long-distance dispersal (LDD). This will be accomplished by addressing three specific sub-hypotheses to test how the frequency of susceptible hosts, degree of host susceptibility, and application of fungicide at the outbreak focus influence subsequent epidemic spread. Methods: The wheat stripe rust disease will be used as a model system in two experiments conducted over four years. In both experiments, the artificially inoculated area ("outbreak focus") at the center of each experimental plot will be treated either the same or different from the remainder of the plot to determine whether initial conditions at plot center have the dominant influence over subsequent disease spread. In the first experiment, three host genetic variables that influence pathogen reproduction (monoculture of a susceptible host genotype, a mixture of 25% of the susceptible genotype with 75% of a resistant genotype, and a partially resistant genotype) will be planted in varying combinations of plot center and the remainder of the plot. In the second experiment, all plots will be planted to a monoculture of the susceptible genotype, and a highly effective fungicide will be used to curtail or "cull" disease at varying times after the beginning of the first infectious period. This will be done either in the outbreak focus only or in an area including both the outbreak focus plus three additional focus widths on each side of the artificially inoculated area. For both experiments, disease prevalence (% of maximum number of infections in each sampling unit) will be recorded weekly in the outbreak focus and at varying distances both downwind and upwind of the outbreak focus. Generality of the results will be evaluated by altering variables in a spatially explicit simulation model, by evaluating relevant data of other diseases in the literature, and through a collaboration to compare field and modeling results for wheat stripe rust with those of foot-and-mouth disease of livestock.